Acetophenone reduction

Simultaneously the wave in increases, and in the final product, corresponding to a mixture of benzaldehyde and acetophenone, only waves 3 and 4 persist. The wave at intermediate stages of the reaction therefore corresponds to the reduction both of the ketol VII and of acetophenone, which is reasonable since the polarographically active benzoyl grouping is the same in both substances. To determine the ketol concentration it is necessary to subtract from wave in the current corresponding to acetophenone reduction. Since acetophenone is formed at the same concentration as benzaldehyde and because for equimolar solutions of benzaldehyde and acetophenone the ratio of wave heights iz/ii is practically equal to unity, it is possible to determine the ketol concentration simply from the difference 4 — t3. 

In the case of acetophenone reduction, it appears that amino alcohols that are sterically hindered at the carbon adjacent to the alcohol lead to much better results. A dramatic effect has been found in the case of prolinol and diphenylprolinol the enantiomeric excess increases from 50% to 97%. Cyclic amino alcohols where the nitrogen is in a 4- or 5-membered ring have exceptional catalytic properties and lead to very good enantiomeric excesses (Tables 16.1 and 16.2). Diphenylprolinol (4) is a very good choice because of its availability and performance. 

Scheme 4.65 Acetophenone reduction under hydrogen transfer conditions.

It is interesting to note that chromium ions have been used as promoters in Raney nickel based catalysts for the selective acetophenone reduction. The selectivity increase in the hydrogenation of the C=0 bond of acetophenone over both chromium ion promoted Raney nickel and Ru-Cr-B/Si02 catalysts was attributed to the strong interaction of the oxygen atom of the carbonyl with Cr ions. All these results suggest the formation of metal ion - metal nanocluster ensemble sites also in nickel-containing catalysts. 

The related ACTC 18 with a phosphinite unit yielded a product with only 18% ee in acetophenone reduction. This result was explained assuming unfavourable steric interactions which result in partly reversed coordination preferences. However, electronic reasons also have to be taken into account because an increase in electron density on the complexed arene ring will undoubtedly affect the electronic nature of the amine and thereby sHghtly decrease the Lewis acidity of the complex as a whole. 

In homogeneous eatalysis using chiral diamine 18 complexed with Rh, the acetophenone was reduced quantitatively with 55% ee, in 7 days. In the case of polymerized complex 36a, acetophenone reduction leads to 33% ee and with its templated analog 43% ee. With 36b, an increase of about 20% ee is observed between polymerized and templated ligand. These increases in ee were ascribed to a favonrable molecnlar imprinting effect of the PM, creating chiral pockets within the polymer network. 

The following example may be considered in order to illustrate the principles involved. Acetophenone reduction in acid medium follows the pathway... 

In the case of monomerization, the form of the current-potential curve and the values of peak current and potential as a function of sweep rate provide a useful means of characterizing this mechanism. In the case of the dimerization type of reaction, however, the i-E curves are not appreciably affected by the chemical reaction. Also, unless the dimerization is relatively slow, the detection of an intermediate in the anodic direction corresponding to a reversed scan is difficult. With acetophenone reduction, for example, reoxidation of the ketyl radical intermediate formed in the cathodic direction of sweep in acid solutions (CHs-COH -c -h e CH3 COH < ) is not observed. 

Catalysts prepared in situ from [Rh(norbornadiene)Cl]2, (+)-diop (30), and triethylamine, give significantly increased enantioselectivity in aryl ketone hydrogenation when benzene is used as solvent rather than methanol the optical yield in acetophenone reduction, for example, is raised from 53% to 80%. 

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